Since at least as early as the mid-1960's, scientific and economic forecasters have been predicting an approaching era of diminishing availability of petrochemical resources to produce the energy and chemical materials needed by industrialized societies. On one hand, discoveries of new petroleum reserves and new petroleum production technologies (e.g., deep-water, off-shore drilling) have staved off an economically catastrophic shortage of crude oil. On the other hand, rapidly industrializing national economies (most notably China and India), coupled with political instability in petroleum-producing regions (most notably the Middle East, Nigeria, and Venezuela), have pushed oil prices to record levels. In January of 1999, crude oil cost $17 per barrel. By early 2006, the price of a barrel of crude oil topped $70 for the first time in history. In March of 2008, the price of a barrel of crude oil topped $110, on its way to an all-time peak of $147 per barrel in July of 2008. As of May 2011, the spot price of a barrel of crude oil is hovering at slightly over $100 per barrel. Environmental, ecological, and political considerations have also effectively made certain proven reserves of petroleum off-limits to commercial exploitation. For example, production of petroleum from proven reserves in the Arctic National Wildlife Refuge in Alaska has been (and for the foreseeable future, will continue to be) blocked by federal and state legislation to preserve this unique natural landscape from human encroachment. Likewise, new off-shore drilling in the Gulf of Mexico has, at least for the present, come to a standstill in the wake of the April 2010 BP/Deepwater Horizon oil spill.
The rippling effect of high crude oil prices on national economies is profound. Not only are gasoline and diesel the principal transportation fuels worldwide, crude petroleum also yields a vast array of chemicals that are feedstocks for an equally vast array of products, from plastics to pesticides. Thus, high crude oil prices spur worldwide price inflation as producers pass on their increased costs of production to consumers.
The economic difficulties caused by increasing demand coupled with diminishing supply is driving efforts to develop alternative and sustainable ways to meet energy and raw material needs. The Roadmap for Biomass Technologies in the United States (U.S. Department of Energy, Accession No. ADA436527, December 2002), authored by 26 leading experts, has predicted a gradual shift from a petroleum-based economy to a more carbohydrate dependent economy. This official document predicts that by 2030, 20% of transportation fuel and 25% of chemicals consumed in the United States will be produced from biomass. Such a shift away from petroleum-based technologies requires developing innovative, low-cost separation and depolymerization processing technologies to break down the highly oxygen-functionalized, polysaccharide molecules found in raw biomass to yield useful bio-derived materials and fuels. In short, abundant biomass resources can provide alternative routes for a sustainable supply of both transportation fuels and valuable intermediates (e.g., alcohols, aldehydes, ketones, carboxylic acid, esters) for production of drugs and polymeric materials. However, unless these alternative routes can be implemented at a production cost roughly comparable to the corresponding production cost when using petroleum feedstocks, the transition will inevitably be accompanied by severe economic dislocations. It is not enough that the transition can be accomplished; to avoid economic upheaval, the transition must be accomplished in an economically feasible fashion.
Furan derivatives (such as hydroxymethylfurfural (HMF), furfural (Fur) and furfuryl alcohol (FurA)) derived from renewable biomass resources have potential as substitutes for petroleum-based building blocks used to produce transportation fuels, plastics, and fine chemicals. For example, levulinic acid (LA) and levulinic acid esters can be produced directly from HMF (Girisuta et al., Green Chemistry 2006, 8, 701-709) and/or indirectly from furfural, via previous hydrogenation of furfural (Sitthisa et al. J. Cat. 277, 2011, 1-13, Merlo et al. Catalysis Communications 10, 2009, 1665-1669) to furfuryl alcohol (Lange et al. Chemsuschem 2009, 2, 437-441; Zhang et al. Chemsuschem 2011, 4, 112-118). LA is a valuable “platform” chemical for fabricating a host of downstream compounds. Furfural is also a key chemical for the commercial production of furan (via catalytic decarbonylation) and tetrahydrofuran (via hydrogenation), thereby providing a biomass-based alternative to the corresponding petrochemical production route (via dehydration of 1,4-butanediol). HMF can be used to produce 5-hydroxymethylfuranoic acid by oxidation of the formyl group or 2,5 dimethylfuran by hydrogenolysis.
Furfural is primarily used in refining lubricating oil. Furfural is also used in condensation reactions with formaldehyde, phenol, acetone or urea to yield resins with excellent thermosetting properties and extreme physical strength. Methyl-tetrahydrofuran (MeTHF), a hydrogenated form of furfural, is a principal component in P-series fuel, which is developed primarily from renewable resources. (“P-series fuel” is an official designation promulgated by the U.S. Dept. of Energy for a fuel blend comprised of pentanes, ethanol, and biomass-derived MeTHF. See 10 CFR §490.) Producing furfural from biomass requires raw materials rich in pentosan, such as corncobs, oat hulls, bagasse, and certain woods (like beech). Even today, most furfural production plants employ batch processing using the original, acid-catalyzed Quaker Oats technology. This technology was first implemented in 1921 by Quaker Oats in Cedar Rapids, Iowa as a means to realize value from the tons of oat hulls remaining after making rolled oats. For an exhaustive history on the production of furfural, see K. J. Zeitsch, “The Chemistry and Technology of Furfural and its Many By-Products,” Elsevier, Sugar Series, No. 13, © 2000, Elsevier Science B.V.) This batch processing results in yields less than 50%, and also requires a large amount of high-pressure steam. The process also generates a significant amount of waste.
Various researchers have tried dehydration of xylose into furfural using acid catalysts such as mineral acids, zeolites, acid-functionalized Mobile crystalline materials (MCM's) and heteropolyacids. Moreau et al. has conducted the reaction in a batch mode using H-form faujasites and a H-mordenite catalyst, at 170° C., in a solvent mixture of water and methylisobutylketone (MIBK) or toluene (1:3 by vol) with selectivities ranging from 70-96% (in toluene) and 50-60% (in MIBK) but at low conversions. Dias et al. showed that a sulfonic acid-modified MCM-41-type catalyst displayed fairly high selectivity to furfural (˜82%) at high xylose conversion (>90%) with toluene as the extracting solvent for the reactions carried out 140° C. In the patent literature, see, for example, U.S. Pat. Nos. 4,533,743 (to Medeiros et al.); 4,912,237 (to Zeitsch); 4,971,657 (to Avignon et al.); and 6,743,928 (to Zeitsch).
5-hydroxymethylfurfural (HMF) is of interest as a lignocellulose (C6-derived) platform chemical which offers multiple upgrading strategies to various intermediates, polymer feedstocks, specialty chemicals, and transportation fuels. Progress in HMF production via sugar (generally glucose or fructose) dehydration has been compiled in several reviews. See M. S. Feather, J. F. Harris, R. S. T. a. D. Horton, Dehydration Reactions of Carbohydrates, in: Advances in Carbohydrate Chemistry and Biochemistry, Academic Press, 1973, pp. 161-224; B. F. M. Kuster, Starch-Starke, 42 (1990) 314-321; J. Lewkowski, Arkivoc, 2 (2001); and X. Tong, Y. Ma, Y. Li, Applied Catalysis A: General, 385, 1-13.)
Multiple systems for fructose dehydration have been proposed, and these systems are conceptually extensible to glucose sugars, which may be isolated from cellulose. Alternatively, glucose produced through cellulose hydrolysis may undergo isomerization via acid-, base-, or enzyme-catalyzed reactions to yield fructose, which may be processed in any of the following systems. The simplest is dehydration of fructose in aqueous media using either solid or homogeneous mineral acid catalysts. Unfortunately, HMF is sufficiently reactive under such conditions such that it undergos rehydration to form levulinic and formic acids. (B. F. M. Kuster, H. M. G. Temmink, Carbohydrate Research, 54 (1977) 185-191.) Additionally, HMF may undergo condensation reactions either with itself or other polyoxygenates to form insoluble humins, limiting HMF yields. (B. Girisuta, L. P. B. M. Janssen, H. J. Heeres, Green Chemistry, 8 (2006) 701-709. B. Girisuta, L. Janssen, H. J. Heeres, Industrial & Engineering Chemistry Research, 46 (2007) 1696-1708.) The problem of levulinic acid formation may be alleviated by processing fructose in non-aqueous solvents, wherein rehydration of HMF to form levulinic acid is less likely. Of the multiple non-aqueous solvents considered (A. Corma, S. Iborra, A. Velty, Chemical Reviews, 107 (2007) 2411-2502.), DMSO has shown the most promise, with HMF yields in excess of 90% achieved using acidic resins. A disadvantage of non-aqueous processing is the low solubility of sugars in organic solvents, which limits sugar loading and thus the large-scale applicability of the technology.
Biphasic reactors for fructose dehydration have been exploited for a number of years, and both homogeneous and heterogeneous acid catalysts have demonstrated promise. For example, Moreau and co-workers demonstrated that biphasic water-MIBK systems coupled with zeolites (i.e., mordenite) of varying Si/Al ratios could achieve high HMF selectivities (>90%), at moderately high conversions (76%) of fructose (C. Moreau, R. Durand, S. Razigade, J. Duharnet, P. Faugeras, P. Rivalier, P. Ros, G. Avignon, Applied Catalysis A: General, 145 (1996) 211-224). Such systems, however, are limited by a sparse partitioning of HMF into the organic phase (MIBK) which restricts HMF concentration in the extracting solvent and necessitates large quantities of solvent
Abundant biomass resources are a promising sustainable supply of valuable intermediates (e.g., alcohols, aldehydes, ketones, carboxylic acids) to the chemical industry for producing drugs and polymeric materials. In this context, the high content of oxygenated functional groups in carbohydrates, the dominant compounds in biomass, is an advantage. This is in contrast to the drawbacks of such functionality for the conversion of carbohydrates to fuels. However, there remains a long-felt and unmet need for efficient processes to selectively remove excess functional groups and to modify other functional groups to create commercially desirable products from biomass.